This invention relates to a photosensitive resin composition solution. More particularly, the present invention relates to a photosensitive resin composition solution for formation of a permanent mask for printed board.
As the photosensitive resin composition for formation of permanent mask of printed wiring board, there have been already known various compositions, such as the photosensitive resin composition composed mainly of an acrylic polymer and a photopolymerizable monomer (Japanese Provisional Patent Publications No. 56018/1978 and No. 1018/1979), the photosensitive resin composition composed mainly of an acrylic polymer, a photopolymerizable monomer and an epoxy resin (Japanese Patent Publications No. 43091/1977 and No. 43092/1977) and the photosensitive resin composition composed mainly of an epoxy resin to which photoreactivity is imparted (Japanese Provisional Patent Publications No. 82073/1979, No. 62636/1983 and No. 132947/1986). Among these, with respect to solder heat resistance and developability, the photosensitive resin composition disclosed in Japanese Provisional Patent Publication No. 132947/1986 is preferred for permanent mask.
On the other hand, the methods for forming photosensitive layers on substrates by use of photosensitive resin compositions may be classified into (a) the film method [the method in which a photosensitive material shaped in uniform film obtained by coating and drying a photosensitive resin composition solution on a support such as polyethyleneterephthalate film, etc. is pressure adhered onto a substrate by using a laminator, etc. to form a photosensitive layer on the substrate] and (b) the direct coating method [the method in which a photosensitive resin composition solution is coated and dried onto a substrate by means of a coating device (e.g., curtain flow coating device, roll coating device, etc.) to form a photosensitive layer on the substrate].
According to the above method (a), a photosensitive layer with uniform film thickness can be formed on a substrate simply without requiring special trained skill. However, when the circuit thickness of the substrate is large, it is difficult to embed sufficiently the photosensitive layer at the portion between circuit and circuit, whereby bubbles may remain between the circuits. For avoiding this problem, it becomes necessary to use a thick film photosensitive material and further to effect pressure adhesion onto the substrate under vacuum. For this purpose, a special and expensive device (e.g., vacuum laminator) is required.
In contrast, in the above method (b), the device is inexpensive and a relatively thinner photosensitive layer can be easily formed on a substrate, but trained skill is required for forming a thick photosensitive layer uniformly. Also, in the case when the circuit thickness of the substrate is large, although bubbles between the circuits in the film method will be avoided, there ensues another problem. That is, due to flowing of the resin composition solution from the circuit upper portion, flowing into the portion around through-hole, the photosensitive layer film thicknesses on the circuit and around the through-hole become thinner (this is called "poor covering"), whereby such portions are susceptible to damage in the subsequent soldering treatment, etc.
The methods for solving the problem in the above method (b) are proposed in Japanese Provisional Patent Publications No. 82073/1979 and No. 62636/1983.
More specifically, in these published specifications, in forming a permanent mask on a substrate by coating a substrate with a specific liquid resin composition solution by use of a curtain flow coater to form a photosensitive layer on the substrate, the viscosity of the resin composition solution provided for coating is described as 500 to 1200 mPas in the case of a composition containing no filler (Japanese Provisional Patent Publication No. 82073/1979), and 200 to 700 mPas in the case of a composition containing a filler (Japanese Provisional Patent Publication No. 62636/1983), and the problem of covering as described above is stated to be solved by such methods.
However, when such proposed methods are applied for the direct coating method of the resin composition composed mainly of a resin containing a polymerizable vinyl group and a thermally reactive epoxy group as disclosed in Japanese Provisional Patent Publication No. 132949/1986, it has been found that no satisfactory result can be obtained for the problem of covering. That is, in the roll coating method or the curtain flow coating method of the resin composition solution, remaining of bubbles frequently occurred between the circuits on the substrate with a highly viscous solution of high concentration while there was the problem of covering insufficiency of the circuit and flowing into through-hole with a solution of low concentration and low viscosity. Thus, no viscosity region could be found where none of these problems occurred and good coating could be effected stably.